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Security and
Authentication
Authentication and
Security
• A major problem with computer communication –
Trust
• Who is sending you those bits
• What they allow to do in your system
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Authentication
• In distributed systems, services are rendered in
response to incoming messages.
• It is important that the server know for sure who the
client is!
• The simple solution is to send the user name and
password with every request
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Kerberos
Authentication in Distributed Systems
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Kerberos History
• Developed at MIT in early 1980’s
• Computing shift from mainframes to workstations
• Pools of distributed workstations connected to
servers
• Concept of “Network Credentials”
• Two commercial and non-compatible versions V4
and V5
• Principles and systems are relevant until today
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Kerberos
• Authentication service, based on a secure
authentication server and on encryption
• The server knows all passwords, but they are never
transmitted across the network
• Passwords are used to generate encryption keys.
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Kerberos Environment
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Kerberos Environment
Separation between two actions:
– Authentication – logging into the “network”
– Communication – holding a session between two
parties
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Kerberos Architecture
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Kerberos Protocol
• The client workstation where the user is trying to
log in sends the user name U to the server.
• The Kerberos server does the following:
– It looks up the user’s password p, and uses a one-way
function to create an encryption key Kp from it.
– It generates a new session key Ks for this login session.
– It bundles the session key with the user name: {U,Ks}.
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Kerberos Protocol
(cont.)
– It uses its own secret encryption key Kk to encrypt
this.
– It bundles the session key with the created
unforgeable ticket, creating {Ks, {U,Ks}Kk}.
– Finally, the whole thing is encrypted using the userkey that was generated from the user’s password,
leading to {Ks, {U,Ks}Kk}Kp.
This is sent back to the client.
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Kerberos Protocol
• The client does the following steps:
– It prompts the user for his password p, immediately
computes Kp, and erases the password.
– Using Kp, the client decrypts the message it got from
the server, and obtains Ks and {U,Ks}Kk.
– It erases the user key Kp.
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Now What?
• Now, the client can send authenticated requests to the
Kerberos server
• Each request is composed of two parts:
– The request itself, R, encrypted using Ks,
– The unforgeable ticket.
• The server decrypts the ticket using its secret key Kk,
and finds U and Ks
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But…
• An eavesdropper can copy the whole request message
and retransmit it
• The Kerberos server does not provide any real services.
All it does is to provide keys for other servers.
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Finally
• Kerberos will send the allocated key Kf to the client
encrypted by Ks, and also send it to the file server using
Kb
• The client will then be able to use Kf to convince the
file server of its identity
– perform operations on files
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Introduction to
Security
Based on Slides by Shlomo Kipnis,
Introduction to Security Course
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What is Security?
• Making sure that bad things do not happen
• Reducing the chances that bad things will happen
• Lowering the impact of bad things
• Providing means to recover from bad things
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Security Challenges
• Securing a variety of different systems
• Securing interfaces between different systems
• Different security goals and needs
• Attackers seek weakest link in the system
• Security people must protect all links in the system
• Maintaining system usability
• Keeping security costs under control
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Threats & Attacks
• Unauthorized access
• Data theft
• Denial of service
• Data destruction
• Computer viruses
• Program manipulation
• Trojan horses
• Information loss
• Data leaks
• Data manipulation
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Eavesdropping and
Packet Sniffing
• Description: Acquiring information without changing it
• Means: Packet sniffers, routers, gateways, capturing and
filtering out packets
• Threats: Sniffing can be used to catch various
information sent over the network
– Login + Password
– Credit card numbers
– E-mails and other messages
– Traffic analysis
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Snooping
• Description: Acquiring information without modifying it
• Means: Browsing documents on disk or main memory
– Using legitimate privileges (insiders)
– Hacking into a system (outsiders)
– Stealing laptops
– Monitoring keyboard strokes
– Observing timing information (covert channels)
• Threats:
– Obtaining sensitive information (files with credit
card numbers)
– Discovering passwords, secret keys, etc.
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Tampering
• Description: Modifying or destroying stored data
• Means: Insiders misusing privileges or outsiders
breaking into system
• Threats:
– Change records – school grades, prison records, tax
payers’ debts (NY $13 million property tax fraud)
– Erase audit trails (by hacker)
– Plant Trojan-horses for password gaining, and other
uses
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Spoofing
• Description: Impersonating other users or computers
to obtain privileges
• Means:
– Account stealing, password guessing, social
engineering
– IP spoofing: E-mail forging, false IP From address,
hijacking
– IP connections
• Threats:
– Forged messages ( “exam is cancelled”)
– Denial of Service (IP attacks, SYN attacks,
Ping-of-Death)
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Jamming
• Description: Disabling a system or service
• Means: Engaging host in numerous (legitimate)
activities until exhausting its resources; spoofing return
addresses to avoid tracing
• Threats:
– Consume all resources on the attacked machines,
e.g., memory (SYN attack), disk (E-mail attack)
– Exploit bug to shut down hosts (ping-of-death)
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Code Injection
• Description: Injecting malicious code to execute on
host with high privileges and infecting other hosts
• Means:
– Virus: attached to executable, spread through
infected floppy disks, E-mail attachments, macros
– Worm: replicate over the Internet
• Threats:
– Everything…
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Methods
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Exploiting Flaws
• Exploit vulnerabilities in software to penetrate systems
– Buffer overflow (e.g., ‘finger’, Internet Worm, Web Site
apps)
– Mobile code security flaws (Java, ActiveX)
• Knowledge spreads faster than remedy
– Hacker bulletins
– Advisories:
• Flaws/fixes repositories, e.g., CERT
• Publicly available software kits to detect known
vulnerabilities, e.g., SATAN, ISS
• But they are not always followed readily, and
are often used to the advantage of hackers
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Password and Key
Cracking
• Guessing: family member names, phone numbers, etc.
• Dictionary Attack: systematic search
– Crack: dictionary attack extended with common patterns
• crack is now employed by sys-admins and the passwd
program
• Exhaustive search:
– Crypt-analysis tools evolve continually
– The Internet provides a massively parallel computing resource
• Crypt-analysis, bad generators, timing analysis
• Smart-card cracking via fault injection
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Social Engineering
• Spoofing a “real system”:
– Login screen
– Phone numbers
– ATM story
• Spoofing a “service”:
– Stealing credit card numbers and PINs
– Stealing passwords
• Agent-in-the-Middle Attacks
– Special print of newspaper
– Router, gateway, bulletin boards, etc.
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Buffer Overflow
Based On Slides by Tomer Harpaz
Advanced OS seminar
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Buffer Overflows
• Common
• Stack or heap
• Overwriting control-data or sensitive data
Memory Organization
0xffff
0x0000
Memory Organization
(cont.)
Memory
addresses
Stack Buffer Overflow
Stack Buffer Overflow
(cont.)
Solutions
• Os level
– Exec shield
– Address space layout randomization
– Etc..
• Programmer level
– fgets (not gets)
– strncpy (not strcpy)
– Etc…
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